The Coriolis mass flowmeter is a meter used for directly and precisely measuring the flow of fluid. Two parallel U-pipes are arranged in the main body of the typical Coriolis mass flowmeter and allowed to be inversely and synchronously vibrated at a resonant frequency thereof, that is, they may move close to or away from each other at the same time. The fluid entering and flowing through the vibrating pipes subject to synchronous vibration will be forced to be vibrated together. To resist against the forced vibration, the fluid gives an reacting force perpendicular to the flow direction thereof on the vibrating pipes to establish a Coriolis effect, leading to the distortion and deformation of the vibrating pipes and the vibration time difference between the fluid input pipe section and the fluid output pipe section, which is called phase time difference. The difference is proportional to the mass flow of fluid flowing through the vibrating pipes. If the time difference may be measured, the mass flow may be determined accordingly. The Coriolis mass flow meter is manufactured based on such principle.
At present, the instruments may be divided into single-pipe type and double-pipe type by number of vibrating pipes in the sensor. Without diverter, the single-pipe type has equal flows throughout the measuring pipe, easy cleanness and good stability in zero point; however, the single-pipe type is vulnerable to external vibration and only applied by some early products and small-diameter instruments. The double-pipe type realizes the measurement of phase difference between pipes, increases the signal, enhance the linearity, and reduces the influence of external vibration at the same time.
The Coriolis mass flowmeter may be roughly divided into straight-pipe type and elbow-pipe type by the pipe shape structure of a sensor. The straight-pipe type is hard to deposite gas and has a compact and light flow sensor. However, the signal is difficult to detect due to the high self resonant frequency. To limit the self-resonant frequency, the pipes are relatively thin-walled and susceptible to wear damage and corrosion. The elbow-pipe type instrument is provided with a low-rigidity measuring pipe to generate a relatively large signal, and the technology is relatively mature. A thick-walled pipe may be used because of low self-resonant frequency (80-150 Hz). The instrument has relatively good wear resistance and corrosion resistance, and has additive errors caused by easily deposited gas and residues and the requirements for installation space.
The double Π-shape pipe structure is a relatively mature structure in the market. The pipe structure has been the most economical sensor structure, characterized by simple structure, easy processing, moderate sensitivity and strong impact resistance.
However, most of the mass flowmeters applied to the food and medical field are not of the double Π-shape pipe structure. The reasons are shown as follows: the flowmeter for measuring should be free of diverter to meet the sanitary requirements of the food and medical field; and, if a Π-shape pipe only comprises a single pipe, the multi-mode coupling may occur due to the complexity of the internal pipe and affect the performance. Therefore, the mass flowmeters used in the food and medical field should be in single-pipe structure or non-Π structure, which reduces the measurement accuracy and hinders the promotion of the mass flowmeter.
To solve such problem, a Coriolis mass flowmeter having a double Π-shape single-pipe sensor without diverter structure is provided according to the prior art. For example, the Chinese patent No. CN1116588C discloses a Coriolis mass flowmeter comprising a continuous fluid flow pipe, wherein the fluid flow pipe has a double-loop pipe, an input pipe for receiving fluid at the flow-in position, a output pipe for returning the fluid to downstream, and a shell enclosing the double-loop pipe; the flowmeter assembly comprises: a second loop having a first end and a second end and arranged on the fluid flow pipe, wherein the fluid material is received from the second end of the first loop through the first end, and led to the output pipe; a crossover section of the fluid flow pipe, wherein the crossover section leads the fluid from the first loop to the second loop; a fixed connection component fixedly connected to the shell and the fluid flow pipe; and a supporting rod fixedly connected to the first loop and the second loop. According to the full contents disclosed in such patent document, the fixed connection component is mainly used to 1. firmly weld the double-loop pipe to the shell; 2. reduce its minor distortion caused by welding and reduce the impact on double-loop pipe by the distortion with considerable mass; and 3. isolate the vibrating component of the flowmeter and the non-vibrating component installed on the fluid flow pipe.
However, the technical solution of the patent document has the following defects in practical use: 1. for minor distortion caused by welding, the considerable mass of the patent document may not completely eliminate but reduce the impact of the distortion; 2. because the fixed connection component is directly welded to the shell base, and the shell base and the shell cover are made of thick-walled material and welded together, the fixed connection component is rigidly connected to the overall shell and has limited effectiveness in vibration isolation; since the fixed connection component may not eliminate all distortions, some vibration of the vibrating component failing to be counteracted symmetrically releases and leads to unstable zero point and metering performance.